Cell cultures of a murine carcinoma known to secrete insoluble components of basement membrane have been studied for ability to secrete collagenous protein. Two collagenous components with characteristics of basement membrane collagen are consistently found in the nutrient medium. The larger is thought to exist as a high molecular weight complex of a collagen with another basement membrane glycoprotein. The smaller (155,000 daltons) could be the collagen part of the complex. It is proposed to isolate and characterize further the smaller collagen component and the larger complex by sizing, ion-exchange and/or affinity chromatography followed by amino acid analysis. The relationship of the smaller collagen component and the complex to one another and to insoluble basement membrane will be assessed by following the fate of radioactively labeled components with time in nonradioactive cultures. Insight as to the molecular structure of basement membrane is expected to be gained so that application to pathologic change in basement membrane will be possible.